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What are the black streaks?

Often mistaken as mold or mildew, the black discolouration and streaks visible on many roofs across Canada is actually algae. The algae can form where moisture tends to dwell on roof surfaces. Humid climates create the most conducive conditions for algae growth.


The most common type, gloeocapsa magma or blue green algae, protects itself from damaging ultraviolet rays by producing a dark pigmented sheath. The algae discoloration typically begins as small spots then quickly transforms into streaks on the roof plane. By the time the algae is visible, it likely has been present for several months or longer.



Algae spores are carried by the wind or by animals and can quickly spread from one rooftop to another. That is why it is common to see algae growth on rooftops throughout an affected neighborhood. The same is true for apartment or condominium complexes, townhouses or row houses.


There is no scientific evidence that algae is damaging to asphalt shingles, although it certainly affects the aesthetics of a roof. On highly reflective or “cool” roofs, algae will undoubtedly have a negative impact on the long-term effectiveness of these roof systems. That is why it is important to address an algae problem as soon as possible.


Moss is a non-vascular plant that obtains water through its leaves, unlike most plants that gather water through its root system. Moss must therefore have a moist environment to survive. In North America, moss tends to grow on north facing roof planes that receive less direct sunlight and stay damp longer than south facing planes. Overhanging tree branches provide additional shade and drop debris on the roof that further holds in moisture and acts as a food source for moss.


Moss spores may also be carried by wind or animals and can easily spread throughout neighborhoods. Unlike algae, moss can be detrimental to asphalt shingle performance. Moss can cause the leading edges of the shingles to lift or curl, which increases the risk of shingle blow-off during wind events. In severe cases, moss build-up can cause lateral water movement resulting in moisture damage to the roof deck or may even cause leaks.


The most effective method of cleaning algae and moss from a roof is Softwashing. We apply a chemical solution at low pressure and allow the solution to dwell. Extended dwell times may be necessary. Algae will disappear and wash away with subsequent rains. Moss will loosen over time and eventually disappear.


There are some steps that can be taken to discourage moss and algae growth:


· Trimming tree branches back to allow more sunlight to reach the roof and minimize debris accumulation will provide an environment less inviting for such growth

· Debris that does begin to accumulate on the roof should be cleared regularly with a leaf blower or similar, non-abrasive method as part of a regular maintenance program.

· Air flow should be directed down the slope to avoid driving debris under the edges of the shingles.

· Keep gutters clean to promote proper water drainage. Do not allow gutters from an upper roof to drain directly on a lower roof; extend the downspout from the upper roof into the lower gutter.


Algae and moss growth on roofs, at a minimum, is an unsightly nuisance. Moss build-up can lead to roof damage or shingle blow-off if left unchecked. Keeping branches trimmed back from the roof and removing any debris that begins to accumulate should be part of a regular roof maintenance program. If moss or algae becomes visible contact us today. 

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